The timeout limit for changing votes is a known feature and designed feature, and as for the reason for it Jeff addressed that in comments on his answer here.
Now there isn't a problem for voting more than one answer that you think is good. The "best" answer is accepted, and there can only be one; however you can up vote all the answers you think are good so don't feel the need to retract just because a "better" answer came in.
If you choose to upvote a question and later regret your decision because you realize it's wrong in some way, then leave a comment. The author may address your concern in a way you didn't consider before or may make an edit. Once a post is edited, then you can change your vote.
Finally in regards to the exact time for the timeout, you know... it was changed to 10 minutes, according to Murphy's law from then on out, it would only be the 11th minute you realized you didn't want to do that up vote. ;)